1. Field of the Invention
The present invention relates generally to a wireless communication system, and more particularly to a drive amplifier that is used at transmitting ends of a wireless communication system.
2. Description of the Related Art
As generally known in the art, a Radio Frequency Integrated Circuit (RFIC) manufactured using a Complementary Metal-Oxide Semiconductor (CMOS) process (CMOS RFIC) is inferior in performance to bipolar junction type transistors, and thus is not suitable for a communication scheme requiring high linearity, such as a Wideband Code Division Multiple Access (WCDMA) or Long Term Evolution (LTE) scheme.
In recent years, however, with price competitiveness of the CMOS RFIC having strengthened due to a reduction in the length of a CMOS transistor and the performance of the CMOS RFIC having improved by applying various topologies, the market share of the CMOS RFIC continues to increase.
The CMOS RFIC converts a signal received from a baseband modem into a high-frequency band signal, and transmits the converted signal through an antenna, or converts a signal received through an antenna into a baseband signal and delivers the converted signal to a baseband modem.
The performance of an RFIC transmitter circuit depends on the distortion of data to be transmitted. Thus, when specifying an RFIC transmitter circuit, the linearity of an output stage circuit, the phase noise of a local oscillation (OS) signal, the amount of IQ channel imbalance, the amount of an IQ origin offset (DC offset), and the like are considered. The specification may be defined by an Error Vector Magnitude (EVM), which is a transmission performance indicator important in WCDMA/HSPA, Mobile WiMax, LTE, and similar data transmission schemes.
Linearity of an output stage circuit is closely related to an Adjacent Channel Leakage Ratio (ACLR) characteristic, and thus it can be said that the development of a highly linear circuit is essential for improving the performance of a transmitting end.
One of the performance indicators that is notable in modern mobile communication terminals is current consumption. The lower the current consumption, the longer the battery life. Accordingly, the low-power characteristic is also an important factor for the evaluation of the performance of an RFIC.
FIG. 1 is a circuit diagram illustrating a conventional drive amplifier that is used at the transmitting end of a communication system.
In FIG. 1, a drive amplifier with a common source structure and a differential input/output structure is shown. In particular, the drive amplifier shown in FIG. 1 employs a pseudo-differential structure that has the advantage of a smaller third order non-linear component than a fully differential structure. As shown in FIG. 1, inductors 70, 80 are used as the load to solve a voltage headroom problem. RF signals passing through AC coupling capacitors 50, 60 are applied to the gate terminals of a Metal Oxide Semiconductor (MOS) transistors 10, 20, and bias voltages Vg are applied thereto, via resistors 30, 40. Further, the outputs Vout+, Vout− are connected to a Surface Acoustic Wave (SAW) filter external to the RFIC, and thus is converted into a single ended output.
In order to improve the linearity of an output signal in the conventional drive amplifier of the type described with reference to FIG. 1, the gate voltages Vg of the input MOS transistors 10, 20 must be high. That is, the gate voltages Vg of the input MOS transistors 10, 20 must be higher than the threshold voltage of the input MOS transistors 10, 20 by at least the magnitude of input RF voltages Vin+, Vin−. If the gate voltages Vg of the input MOS transistor 10, 20 are not sufficiently high, the gate voltages Vg are momentarily lower than the threshold voltage and will fall out of the saturation region, which may cause signal distortion.
However, if the gate voltages Vg are raised, then the currents flowing in the input MOS transistors 10, 20 are increased, which means an increase in the current flowing in the whole circuit. As a result, an excessively high gate voltage Vg will increase the current consumption at the transmitting end, resulting in rapid battery drain.